This invention is directed to particle charging devices, and is more particularly concerned with devices of this type having two or more parallel generators for defining a main field, the generators including corona discharge electrodes.
FIG. 1 of this application discloses generally the concept of charging particles by a corona discharge process.
In order to electrically charge solid fine particles, as well as for contact charging and friction charging processes which do not require any special charging device, the form of a charging device illustrated in FIG. 1 is comprised of a needle-like or linear corona discharge electrode 1, a plate-like, cylindrical or circular opposite electrode 2, and a DC power source 3, as shown in FIG. 1. The DC corona discharge 4 enables the bombardment and adhesion of the ions 5 caused by the corona discharge onto solid fine particles 6 in corona space 8. This known method has the great disadvantage that the magnitude of charging quantity received by particles cannot be clearly determined, and therefore the control of the quantity of charged particles is technically difficult. In the method of using corona discharge as shown in FIG. 1 there is a clear quantitative relationship that the saturation charge on charged particles 7 is generally proportional to the square of particle diameter and to the field intensity of charging area. The charged particles 7 are driven by the coulomb force from the corona discharge electrode 1 to the opposite electrode 2. This method has the disadvantage that particles with large charges adhere to the surface 9 of the opposite electrode and are not supplied to the intended working area 10, while only particles with small charges are supplied to area 10.
In order to overcome the disadvantage, I have disclosed in Japanese Patent Application No. 71-50087, filed July 7. 1971, a method of the type, for example, shown in FIG. 2 of the present disclosure, wherein alternating electrodes 11 and 12 are positioned parallel to one another at a determined spacing. The electrodes, are insulated from one another, small holes or fine openings 13 and 14 respectively are provided in the alternating electrodes and discharging 3rd electrodes 15 and 16 respectively are provided in the centers of the holes or openings. The electrodes 15 and 16 are insulated from the alternating electrodes. An alternating voltage is applied between said alternating electrodes 11 and 12, for example, by the secondary winding terminals 18 and 19 of a transformer 17, to form an alternating field in the charging space 20 between the electrodes. An alternating voltage is also supplied, as illustrated, to the discharging 3rd electrodes 15 and 16, for example by the secondary winding terminals 27 and 28 of transformer 17 connected to rectifiers 21 and 22, series resistances 23 and 24 parallel capacitors 25 and 26. In the arrangement illustrated in FIG. 2, when the alternating electrode 11 has a sufficiently high negative voltage, spark discharge is generated repetitively only between this electrode and the 3rd electrode 15. At this time, no discharge occurs between electrodes 12 and 16. The spark discharge generates plasma containing both positive and negative ions between both the electrodes, and of these, the negative ions are emitted into the charging space 20 from left to right for the bombardment charging of solid fine particles 6. When the electrode 12 has a sufficiently high negative voltage, with the polarity reversed, negative ions are emitted into the charging space 20 from right to left in accordance with the above discussed principle, for the same bombardment charging of the particles, so that large charges, negative in this instance, are applied to the particles. The colomb force acting on the particles alternates to prevent adhesion of particles onto the electrodes.
However, while the above arrangement is structurally very simple, it has been found to have a disadvantage that, when the atmosphere contains combustible gas or combustible powder, the energy of the spark discharge generated repetitively between discharging 3rd electrodes and alternating electrodes may cause ignition.
In a modification of the concept of the above system, my Japanese patent application no. 48-100901, filed in Japan on Sept. 7, 1973, discloses a system wherein corona discharge is employed, instead of the arc discharging of the former apparatus. The system of this latter disclosure, however, only employs a single corona discharge electrode for each generator. U.S. Pat. No. 4,029,995, based upon Japanese patent application No. 49-78222, filed in Japan on July 10, 1974, discloses a modification of the first two disclosed systems, wherein the generating means comprises planar electrodes each including a plurality of corona discharge electrodes. In the system of Itoh, however, the corona discharge voltage and the main field supply voltage are continuous AC voltages. As a consequence, in accordance with the concept of this reference, it is necessary to provide complex phase shifters for shifting the phase between the plasma producing voltages. In addition, it is necessary to provide a source of voltage for producing the main field that is twice as high as the frequency of the voltage for producing the plasma.
The object of the present invention is to provide a very high performance alternating field type device for electrically charging particles overcoming the above disadvantages, and thus to provide an accurate and highly efficient device for electrically charging particles which is required for various electrodynamic fine particles control devices such as electrostatic precipitators and sorter transporters, and various electrostatic applied devices such as electrostatic hair planters and electrostatic sprayers.
Briefly stated, in the present invention, the above object is attained by using plasma sources arranged substantially in planes, and by employing corona discharge free of ignition or corona discharge through insulators, instead of employing a plasma source with repetitive spark discharge.
The device for electrically charging particles, in accordance with the present invention, comprises two or more parallel plane plasma generating means arranged opposite to one other at a determined spacing as if to hold introduced particles from both sides. The generating means have corona discharge electrodes, and corona generating high voltage power sources as provided for applying a high voltage alternately to the corona discharge electrodes of said respective plane plasma generating means in order to generate corona discharge. A main field forming AC high voltage power source is provided for applying an AC high voltage between the opposing plane plasma generating means, to form an AC main field in the charging space therebetween. The sources are controlled so that a voltage is applied from the corona generating AC high voltage power source to the respective plasma generating means to generate corona discharge, and thereby to generate plasma corona discharge alternately in the adjacent region, only when the plane plasma generating means have a specific polarity. Thereby unipolar ions of this polarity are alternately emitted from the opposing plane plasma generating means into the charging space therebetween, to charge the particles introduced into said charging space, by alternate bombardment by said unipolar ions from both sides in the AC field.
Remarkable action effects can be attained by this arrangement. The particles concerned can be charged perfectly up to a theoretically determined saturation point, with the possibility of ignition completely suppressed even in the presence of combustible gas or combustible powder in the atmosphere. The total amount can be supplied to the working area without adhering to electrodes. In addition, there is a further excellent effect that, since plasma containing positive and negative ions and bipolar ion atmosphere are generated periodically in said plasma generating means, unipolarly charged particles with high electric resistance, if adhering to the component electrodes, are discharged immediately, so that the charging effect is not hindered by generation of inverse ionization.
The system of the present invention thereby utilizes a principle that is entirely different from that of the Itoh reference. Instead of the two continuous AC voltages as employed in the prior art, the system of the present invention employs interrupted AC voltages which are turned on when the corresponding plane electrode has a determined polarity. As a consequence, the switching excitation is simpler and more economical, and does not require the use of an expensive phase shifter and difficult adjustment of such a phase shifter. In addition, in accordance with the invention the main field may be produced by voltages of the mains frequency, and so may be derived much more easily than in the system of this reference. Further, considerable flexibility is obtained by not limiting the relative frequencies of the voltages, as in the arrangement of Itoh.